Which muscle is the primary trunk flexor?

Which muscle is the primary trunk flexor?

• A. External Oblique
• B. Internal Oblique
• C. Erector Spinae
• D. Rectus Abdominis

Answer: (D)

The main action here is trunk flexion—bending the torso forward at the waist. The muscle that runs straight up the front of the abdomen, the rectus abdominis, is positioned right along the midline to pull the rib cage toward the pelvis when it contracts. This straight, vertically oriented muscle shortens directly to bend the spine forward, making it the primary trunk flexor. The external and internal obliques can contribute to flexion, especially when working together and aiding with abdominal compression, but they also handle rotation and lateral bending, so they’re not the main flexors. The erector spinae runs along the back and mainly extends the spine, resisting forward bending.

The main action here is trunk flexion—bending the torso forward at the waist. The muscle that runs straight up the front of the abdomen, the rectus abdominis, is positioned right along the midline to pull the rib cage toward the pelvis when it contracts. This straight, vertically oriented muscle shortens directly to bend the spine forward, making it the primary trunk flexor. The external and internal obliques can contribute to flexion, especially when working together and aiding with abdominal compression, but they also handle rotation and lateral bending, so they’re not the main flexors. The erector spinae runs along the back and mainly extends the spine, resisting forward bending.